Prior to the days of sensitivity with respect to environmental pollution, it was common practice to burn off refinery waste gases at flaming towers or the like or to simply discharge refinery gases into the atmosphere. These gases generally contain sulfur dioxide (SO.sub.2) and H.sub.2 S, together with small amounts of ammonia. In recent years, however, recovery of these substances from the gases before they are discharged into the atmosphere has become a necessity and efforts have been made, therefore, to use the exhaust gases of petroleum refineries in the Claus process for the production of sulfur.
In the Claus process, hydrogen sulfide containing gases are oxidized in a Claus furnace to produce elemental sulfur and SO.sub.2 in accordance with the equations EQU H.sub.2 S + 1/2 O.sub.2 = H.sub.2 O + S (1)
and EQU 2H.sub.2 S + 3 O.sub.2 = 2H.sub.2 O + 2SO.sub.2 ( 2)
The unreacted hydrogen sulfide not utilized in this reaction can be transformed to sulfur in one or more catalytic stages in accordance with the equation EQU 2 H.sub.2 S + SO.sub.2 = 2H.sub.2 O = 3S (3)
these reactions are not fully effective if the Claus process gas contains ammonia. Thus, unless all ammonia is burned in the Claus process it tends to enter the downstream apparatus together with residual H.sub.2 S, and SO.sub.2 so that salts such as ammonium sulfide, ammonium hydrogen sulfide, ammonium sulfite, ammonium hydrogen sulfite and the like form in the equipment and pipe networks. Such salts tend to deposit on cool surfaces of the plant; i.e. on the walls of the pipelines and the reactor, so that free flow of the gas is obstructed.
As the deposits increase, they eventually clog the flow cross section so that the plant eventually becomes inoperative and must be subject to a relatively long downtime as the solid deposits are removed.
Such clogging can be avoided as the ammonia is eliminated by being burned together with the hydrogen sulfide in the presence of a considerable excess of combustion air. However, this reaction, in accordance with equation (2), leads to the burning of more H.sub.2 S to form SO.sub.2 than is desired.
The overcombustion of H.sub.2 S adversely affects the heat balance of the Claus process and disturbs the stoichiometric ratio required for reaction (3). In this reaction two moles of hydrogen pg,4 sulfide are required to react with each mole of SO.sub.2 in a reduction reaction to form elemental sulfur. When H.sub.2 S is consumed in excess because of the need to burn off the ammonia, not only is the sulfur dioxide level increased, but the H.sub.2 S level is reduced and hence the two to one stoichiometry required for reaction (3) cannot be maintained.
This has been recognized heretofore and hence it has been proposed to make up the deficit of reducing gas by adding water gas or coke-oven gases as additional reducing agents for the reduction of NH.sub.3 -containing gas.
These additional reducing agents transform part of the sulfur dioxide to H.sub.2 S as described in German Pat. No. 884,352, thereby tending to restore the proper stoichiometry desired for reaction (3).
However, the addition of extraneous reducing agents as described in the latter patent has been found to be complicated and frequently unreliable being incapable of reproduction because of changing compositions of the reducing gases which are used.
Moreover, many of the reducing acids contain hydrocarbons and tend to form carbon black which deposits in the apparatus and eventually clogs and obstructs the latter, which tends to hinder the catalytic reaction by depositing upon the catalyst, and which tends to darken the color of the end product, i.e. the sulfur.
It has also been suggested (see German published application (Offenlegungsschrift) No. 2,122,000) to burn all of the gas containing the acid components and ammonia, oxidizing the gaseous constituents at elevated temperatures in the presence of a large excess of oxygen. This forms primarily sulfur dioxide, nitrogen and water vapor.
The resulting gas is subsequently combined with a gas which is free from ammonia and contains hydrogen sulfide, the proportions being selected so that H.sub.2 S and SO.sub.2 are present in molar ratio 2:1 which is stoichiometrically required to form sulfur as noted in connection with equation (3) above.
This process depends upon the availability of a gas stream which contains H.sub.2 S and is free from NH.sub.3 and which can be mixed with the gas stream containing sulfur dioxide. Refinery plants seldom have available such gases since practically all the waste gases of a refinery are more or less contaminated with ammonia. The separate production of a pure gas without extraneous components other than H.sub.2 S is too expensive and uneconomical to be practical.